Comtech EF Data SNM-1001 - Manual (Page 35)

Theory of Operation SNM-1001 Network Control Modem
4–4 Rev. 1
From the encoder, the data signal passes through a set of variable-rate digital Nyquist
filters. The filter set is for Comtech EFData Closed Network. There are activity detectors
on both the I&Q channel Nyquist filters.
The digital Nyquist filters are followed by Digital to Analog (D/A) converters and
reconstruction filters. These filters provide proper spectral shaping and equalization. The
filters are under control of the M&C. The symbol rate is 19.2 ks/s.
The I&Q filtered data signals are applied to the RF modulator, which converts them to a
modulated carrier. The spectral shape will be identical to that of the input data streams,
but double-sided about the carrier frequency.
The RF synthesizer provides the proper frequencies to convert the modulator IF to the
desired output frequency in the 50 to 180 MHz range. The synthesizer has multiple
loops, and incorporates a DDS chip to accommodate 100 Hz steps over a range of 130
MHz. The RF section has a frequency stability of
± 1 x 10 -5 .
The signal from the power combiner is sent to the output amplifier, which amplifies the
low level signal from the modulator section to the proper level for output from the
module. The amplifier contains circuitry that provides programmable control of the
output level over a range of -5.0 to -30.0 dBm, in 0.1 dB steps. Power leveling is
provided at
± 1.0 dB to maintain the stability of the output level over time and
temperature.
Fault information from the modulator is sent to the M&C, and includes:
•
Synthesizers out-of-lock
•
RF output leveled
•
Input data clock activity
•
I channel digital filter activity
•
Q channel digital filter activity